No 970 board is good for sli or cfx the first pci-e will be 2.0 16x and the second is 2.0 8x, for whatever reason it had issues and would get stuck at 16x 4x 1.1 as well.
It bottlenecked my 6870's and general did not work well at all .

Secondly SLI/CFX is a gimmick when it does work, it works well in certain games and not so well in many others, you are much better off getting a single better gpu.

I pretty much all the demanding games SLI/CFX works extremely well. In most indie games it doesn't work, but it isn't really needed. So basically SLI/CFX works when it is needed and doesn't when it isn't. They definitely aren't gimmicky.

Is your GTX 660 the 2 GB VRAM? If so then there may be games in the long run (like Watch Dogs) that will limit how high you can crank up the settings even though your SLI GTX 660 OC might have the grunt to handle it.